A compact disc (CD) enables recording of 74 minutes of music data and recording of 650 MB of digital data by using an optical system including a light source having a wavelength of 780 nm and an objective lens having a numerical aperture of 0.45. On the other hand, a digital versatile disc (DVD) enables recording of two hours and fifteen minutes of motion picture based on MPEG2 or 4.7 GB of digital data, by using an optical system including a light source having a wavelength of 650 nm and an objective lens having a numerical aperture of 0.6.
Further, in recent years, high-density and large-capacity optical discs have been increasingly expected because high-definition motion pictures having 1000 or more lines of horizontal resolution have been broadcast, and performances of personal computers have been enhanced. In order to meet the expectation, there has been proposed an optical disc system in which a light source having a wavelength of about 400 nm and an objective lens having a numerical aperture of 0.85 are combined, thereby to realize a recording capacity exceeding 20 GB per side.
As described above, in the conventional optical disc device, an increase in the recording density of data on the disc has been realized by using a light source having a shorter wavelength and an objective lens having a larger numerical aperture. However, higher-density recording by the shorter wavelength and larger numerical aperture is approaching its limit. To be specific, in an area having a wavelength of 400 nm or shorter, since the wavelength dispersion of a glass material used for the lens is increased, it is difficult to control the aberration. Further, when employing a solid immersion lens which is under development for larger numerical aperture, the operating range of the lens, i.e., the distance between the lens and the disc, is extremely short (about 50 nm), resulting in difficulty in replacing the disc.
In order to resolve these problems and further increase the recording density of data on the disc, the holographic recording technique receives widespread attention.
FIG. 7 is a diagram for explaining a shift-multiplexed recording type optical disc system which has been proposed by Psaltis et al., illustrating a schematic construction of the optical disc system.
A shift-multiplexed recording type optical disc system 200 shown in FIG. 7 comprises a laser source 1, a beam expander 7, a half mirror 8, a mirror 10, a spatial light modulator 2, a Fourier transform lenses 3 and 4, a hologram disc 5, a convergence lens 11, and a two-dimensional photoreceptor array 6.
A laser beam emitted from the laser source 1 is increased in its beam diameter by the beam expander 7 and then split by the half mirror 8. The traveling direction of one of the split beams is changed by the mirror 10 so that the beam passes through the spatial light modulator 2. The beam which has passed through the spatial light modulator 2 is converged by the Fourier transform lens 3 and directed onto the hologram disc 5 as a signal light 20. The other beam is converged by the convergence lens 11 to become a reference light 22, and the reference light 22 is directed onto the same position in the hologram disc 5 as the position where the signal light 20 is directed.
The hologram disc 5 has a construction in which a hologram medium such as photopolymer is sealed between two glass substrates, and interference fringes of the signal light 20 and the reference light 22 are recorded in the hologram medium.
The spatial light modulator 2 has an optical switch array in which plural optical switches are two-dimensionally arranged, and the respective optical switches are turned on and off according to an input signal 23. Each optical switch is a cell corresponding to one bit of video information. For example, the spatial light modulator 2 having 1024×1024 cells can display 1M bits of information simultaneously. When the light beam passes through the spatial light modulator 2, the 1M bits of information displayed on the spatial light modulator 2 are converted into a two-dimensional light beam array and then converged by the Fourier transform lens 3.
When playing the recorded signal, only the reference light 22 is applied onto the hologram disc 5. Then, a playback signal light 21 as a diffracted light from the hologram 5 passes through the Fourier transform lens 4 to be converted into a two-dimensional light beam array, and this light beam array is applied onto the two-dimensional photoreceptor array 6. The two-dimensional photoreceptor array 6 comprises photoreceptors which are arranged two-dimensionally, and the arrangement of the photoreceptors corresponds to the arrangement of the optical switches in the spatial light modulator 2. Accordingly, in the two-dimensional photoreceptor array 6, the respective light beams in the two-dimensional light beam array are light-to-electricity converted by the corresponding photoreceptors, and a playback signal 24 is outputted.
The optical disc system 200 shown in FIG. 7 is characterized as follows. That is, since the hologram medium is as thick as about 1 mm, the interference fringes are recorded as thick gratings, i.e., Bragg gratings. Therefore, it is possible to carry out angle-multiplexed recording, i.e., information can be recorded with varying the incident angles of the signal light and the reference light onto the optical disc, resulting in an increase in the amount of information to be recorded on the optical disc. In the optical disc system shown in FIG. 7, the angle multiplexing is realized by shifting the irradiation position of the spherical wave reference light, instead of varying the incident angle of the reference light 22. That is, it is realized by utilizing that the reference light incident angle sensed by each portion of the medium slightly changes when the hologram disc 5 is slightly rotated to shift the recording position.
Next, the operation of the two-dimensional photoreceptor array 6 will be specifically described.
When the two-dimensional light beam array is applied onto the two-dimensional photoreceptor array, a playback signal light spot array in which playback signal light spots are arranged two-dimensionally is formed on the surface of the photoreceptor array.
FIG. 8 shows the layout of the playback signal light spot array on the two-dimensional photoreceptor array. The respective light spots 25 one-by-one correspond to the respective photoreceptor cells 9 in the two-dimensional photoreceptor array 6, and the light intensity of each optical spot 9 is detected by the corresponding photoreceptor cell 6.
In FIG. 8, solid-line circles indicate the playback signal light spots 25 on the respective photoreceptor cells in the state where the positional relationships among the Fourier transform lens 3, the hologram disc 5, and the two-dimensional photoreceptor array 6 are appropriate in the optical disc system shown in FIG. 7. Each light spot 25 is positioned in the center of the corresponding photoreceptor cell 9. Further, dotted-line circles indicate the playback signal light spots 25 on the respective photoreceptor cells in the state where the light source wavelength becomes shorter than the optimum value during playback. As for the position of each light spot 25 in the corresponding photoreceptor cell 9, the farther the photoreceptor cell is apart from the center of the two-dimensional photoreceptor array, the more the corresponding light spot 25 deviates from the center of the photoreceptor cell toward the center of the photoreceptor array.
That is, when the light source wavelength becomes shorter during playback, the diffraction angle of the reference light at the hologram disc 5 becomes smaller, whereby the expanse of the playback signal light spot array becomes narrower. Further, although it is not shown in FIG. 8, when the hologram disc 5 inclines, the direction of diffraction of the reference light changes with the inclination of the disc, and therefore, the playback signal light spot array horizontally shifts from the proper position on the two-dimensional photoreceptor array. Further, when there is an eccentricity of the hologram disc 5, the playback signal light spot array undesirably rotates on the two-dimensional photoreceptor array in agreement with the disc rotation.
When there occurs the deviation of the light source wavelength or inclination or eccentricity of the hologram disc 5 as described above, the position of each light spot 25 on the two-dimensional photoreceptor array shifts and, if the light spot 25 is applied over plural photoreceptor cells, the light intensity of the light spot 25 cannot be correctly detected.
Japanese Published Patent Application No. 2002-216359 discloses a countermeasure against such position deviation.
The technique disclosed in this literature will be described with reference to FIG. 9.
As shown in FIG. 9, with respect to photoreceptor cells 26 at four corners of a two-dimensional photoreceptor array 6a, each photoreceptor cell 26 is divided into two photoreceptor areas 26a, and the position of the light spot 25 with respect to the photoreceptor is detected on the basis of the ratio of the output signals from the photoreceptor areas 261 and 262. Then, according to the position deviation of the light spot 25, the position of the two-dimensional photoreceptor array 6a is adjusted so that each photoreceptor cell of the two-dimensional photoreceptor array 6a follows the displacement of the corresponding light spot 25.
In the technique disclosed in the above-mentioned literature, however, in order to adjust the position of the light spot with accuracy, servo operations in six axes are required for adjusting the positions of the two-dimensional photoreceptor array 6 in the directions of three axes x, y, and z, and the rotation angles around the respective axes, whereby the circuit construction for position adjustment is complicated and, further, the circuit scale is increased.